A quick overview of the ruling: A divided Supreme Court upheld the constitutionality of the Obama administration’s health-care law, in one of the most anticipated high-court rulings in a generation. The court said Congress was acting within its powers under the Constitution when it required most Americans to carry health insurance or pay a penalty—the provision at the center of the two-year legal battle. It upheld the mandate as a tax, in an opinion by Chief Justice John Roberts.

The ruling is a victory for Democrats and President Barack Obama, who had passed the biggest reworking to the health system since the creation of Medicare in the 1960s and faced the prospect of the court nullifying their effort. It also averts disruption for hospitals, doctors and employers who have spent more than two years preparing for changes in the law.

Even as the law’s fate was in doubt, the administration moved ahead with implementing its provisions. It has been negotiating with states to set up exchanges where consumers can buy subsidized insurance policies and sign up millions of lower-income Americans for Medicaid. Some states, including Florida and Texas, refused to cooperate because they expected the law to be overturned.

For Mitt Romney, the high court\’s decision basically means he won\’t have to change his stump speech. Expected to speak later today, Mr. Romney is sure to promise to repeal the law if he\’s elected. It also means he\’ll face less pressure to offer specifics on what he\’d replace the law with — a positive for a campaign that tends to shy away from details.

And more from the WSJ's Brent Kendall

More from inside the court, from our Brent Kendall, who will be going through the ruling line by line: The court’s ruling is largely a victory for the Obama administration. The court’s 59-page opinion rejects the government’s primary argument that Congress’s power to regulate interstate commerce gives lawmakers the authority to require citizens to buy insurance or pay a penalty. But the court goes on to uphold the insurance mandate on other grounds. “It is reasonable to construe what Congress has done as increasing taxes on those who have a certain amount of income, but choose to go without health insurance,” Chief Justice Roberts writes. “Such legislation is within Congress’s power to tax.”